As a positive-working photoresist composition (hereinafter referred to as a "photoresist"), a composition containing an alkali-soluble resin and a naphthoquinone diazide compound as a photosensitive material is generally used. For example, novolak phenol resin/naphthoquinone diazide-substituted compounds are described in U.S. Pat. Nos. 3,664,473, 4,115,128 and 4,173,470; and cresol-formaldehyde novolak resin/trihydroxybenzophenone-1,1-naphthoquinone diazosulfonic acid esters are described in L. F. Thompson, Introduction to Microlithography, No. 219, pp. 112-121 (American Chemical Society).
Since a novolak resin as a binder for a photoresist is soluble in an aqueous alkali solution without being swelled thereby, and provides high durability or resistance to plasma etching when using the images formed therefrom as a mask for etching, the novolak resin is particularly useful for this purpose. The naphthoquinone diazide compound which is used as a photosensitive material functions as a dissolution inhibitor for reducing the alkali solubility of a novolak resin, but is decomposed by irradiation with light to form an alkali-soluble material, which acts to sharply increase the alkali solubility of a novolak resin. Thus, because of the large solubilizing change an exposure to light, the naphthoquinone diazide compound is particularly useful as a photosensitive material for a positive-working photoresist.
Many positive-working photoresist compositions containing a novolak resin and a naphthoquinone diazide series photosensitive material have been developed and practically used, with sufficient results in fabrications of line widths of up to about 1.5 .mu.m to 2 .mu.m.
However, in integrated circuits, the degree of integration has been greatly increased and in the production of a semiconductor base plate required for very large scale integration, etc., the fabrication of very fine patterns having a line width of less than 1 .mu.m has been required. For such a use, a photoresist having a particularly high resolving power, a high pattern form reproducing accuracy capable of accurately reproducing an exposure mask, and a high sensitivity to permit high reproducibility has been required. Conventional positive-working photoresists described above have not satisfied these requirements.